Conventional water absorbent materials include hydrolysates of graft polymers of starch and acrylonitrile, and partially neutralized products of crosslinked polyacrylic acid. Furthermore, examples of water absorbent polyamino acid based resins obtained by hydrolyzing a partially crosslinked product of polyamine polyaspartate have also been disclosed in Japanese Unexamined Patent Application, First Publication, No. Hei 7-309943 and Japanese Unexamined Patent Application, First Publication, No. Hei 9-169840. However, water absorbent materials produced from polyamino acid based resins display insufficient gel strength. Furthermore, although water absorbent materials produced from acrylic resins are typically capable of absorbing between several hundred and several thousand times their own weight of fresh water, this water absorbing ability decreases to an extremely low level for water which contains salts. As a result, in the water absorbent material development field, various trials for enhancing the water absorption capability for water containing salts have been made.
As the water absorbent material having an enhanced water absorption capability for water containing salts, for example, the following suggestions have been made wherein an ionic water absorbent material having a small salt resistance is used in combination with a nonionic water absorbent material having a large salt resistance.
(1) A water swelling polymer comprising a copolymer of an ethylenically unsaturated monomer with a carboxyl group and a base thereof, and a polyoxyalkylene glycol allyl ether with a hydrophobic group at one terminal (Japanese Unexamined Patent Application, First Publication, No. Sho 62-27408); and (2) a water absorbent polymer comprising a copolymer of an ethylene based unsaturated monomer with a carboxyl group and an associated base, and an alkylpolyoxyalkylene glycol mono(meth)acrylate with an alkyl group at one terminal (Japanese Unexamined Patent Application, First Publication, No. Hei 3-93815).
However, in the conventional water absorbent materials described above, although the important characteristics such as the water absorption capability (volume of water absorption, speed of water absorption) and the salt tolerance are improved to some extent, the improvement is not always sufficient.
Furthermore, in order to solve the problems described above, water absorbent materials comprising a copolymer of either a sulfoalkyl (meth)acrylate or an acrylamide (Japanese Unexamined Patent Application, First Publication, No. Hei 10-81714) or a copolymer of a nonionic monomer and acrylic acid (Japanese Unexamined Patent Application, First Publication, No. Hei 9-143210) have also been proposed. However, although these water absorbent materials offer an improved water absorption of water which contains salts, the water absorption for pure water or water with only small amounts of ions actually decreases, and the initial water absorption speed is also slow.
In addition, as an example of a water absorbent material comprising a combination of a polyamino acid and a copolymer comprising a polyacrylic acid, the water absorbing agent composition comprising a polyamino acid and a crosslinked polyacrylate polymer as the main constituents is disclosed in Japanese Unexamined Patent Application, First Publication, No. Hei 7-310021. However, this water absorbent material exhibits little water absorption capability, and even when the surface of the water absorbent resin is crosslinked using a surface crosslinking agent, the gel strength displays no improvement.
An object of the present invention is to provide a water absorbent material which displays superior water absorption of solutions with high concentrations of salts such as sea water, calcium chloride deliquescent solutions, blood, and other body fluids (such as urine and sweat) and further suffers no deterioration in the water absorption ratio or the water absorption speed relative to pure water or water with a low ion content.